The present invention relates to a driving device in which an electromechanical transducer is used and relates to a driving device which is suitable for drive of lenses of a camera, precision drive of an X-Y table, and the like, for example.
Conventionally, there has been known a driving device in which a movable body is moved with utilization of extension and contraction of a piezoelectric element that is an electromechanical transducer. FIG. 1 and FIG. 2 show an example of linear-type driving device. In a driving device 100 shown in FIG. 1, one end of a piezoelectric element 101 with respect to a direction of extension and contraction is fixed to an end surface of a fixed body 102 with adhesive 102a, and a driving rod 103 as a support member for a movable body is fixed to the other end of the element 101 with adhesive 101a. A feeder member 104 is connected to the piezoelectric element 101 with conductive adhesive, and specified pulse voltage is thereby applied to the piezoelectric element 101.
As shown in FIG. 2, a movable body 108 is designed to be slidable along the driving rod 103. The movable body 108 is composed of a slider 105 which is a main element of the movable body 108, a pinch member 106 for pinching and holding the driving rod 103 with the slider 105, and a leaf spring 107 for pressing the pinch member 106 toward the slider 105 with interposition of the driving rod 103. Optical members such as lenses, for example, are mounted to the slider 105 so that the optical members are linearly driven by movement of the movable body 108 on the driving rod 103.
FIGS. 3A-3D show a principle of driving in the driving device 100. Pulse voltage having a sawtoothed waveform of mildly rising sections (between A and B) and steep falling sections (between B and C) as shown in FIG. 3D, for example, is applied to the piezoelectric element 101 of the driving device 100. In the mildly rising sections (between A and B) of the pulse voltage, the piezoelectric element 101 undergoes slow elongation displacement in a direction of thickness thereof as shown in FIG. 3B, and the driving rod 103 fixed to piezoelectric element 101 moves in a thrusting direction. Concomitantly, the movable body 108 engaged frictionally with the driving rod 103 moves together with the driving rod 103.
In the steep falling sections (between B and C) of the pulse voltage, the piezoelectric element 101 undergoes rapid contraction displacement in the direction of thickness thereof, and the driving rod 103 fixed to piezoelectric element 101 is displaced rapidly in a returning direction. On this occasion, as shown in FIG. 3C, an inertial force of the movable body 108 overcomes the frictional force between the movable body 108 and the driving rod 103 and causes a slip, so that the movable body 108 substantially remains in that position without moving. As a result, the movable body 108 moves in the thrusting direction, by a difference in amount of movement between the extension and contraction of the piezoelectric element 101, from an initial state shown in FIG. 3A. With repetition of such extension and contraction of the piezoelectric element 101, the movable body 108 is driven along the driving rod 103 in the thrusting direction.
Contrarily, the movable member 108 is driven in the returning direction according to a principle opposite to that described above. That is, pulse voltage having a sawtoothed waveform composed of steep rising sections and mildly sloped falling sections is applied to the piezoelectric element 101. In the steep rising sections of the pulse voltage, the piezoelectric element 101 undergoes rapid extension displacement and the driving rod 103 fixed to piezoelectric element 101 concomitantly undergoes rapid displacement in the thrusting direction. Then an inertial force of the movable body 108 overcomes the frictional force between the movable body 108 and the driving rod 103 and causes a slip, so that the movable body 108 substantially remains in that position without moving.
In the mildly sloped falling sections of the pulse voltage, the piezoelectric element 101 undergoes slow contraction displacement, and the driving rod 103 fixed to piezoelectric element 101 concomitantly undergoes slow displacement in the returning direction. Then the movable body 108 is displaced together with the driving rod 103 in the returning direction. As a result, the movable body 108 moves in the returning direction from an initial state by a difference in amount of movement between the extension and contraction of the piezoelectric element 101. With repetition of such extension and contraction of the piezoelectric element 101, the movable body 108 is driven along the driving rod 103 in the returning direction.
In the driving device 100, as shown in FIG. 4, the driving rod 103 is composed of a shaft member having a circular cross section, and the pinch member 106 is composed of a bent plate member having end faces shaped like a letter “V”. The pinch member 106 is pressed against the driving rod 103 by the leaf spring 107 having end faces generally shaped like square brackets, and the frictional force is thereby produced between the pinch member 106 and the driving rod 103, and between the slider 105 and the driving rod 103.
In a cross section perpendicular to a movement direction of the movable body 108, a position of the pinch member 106, however, is not restricted with respect to directions along a circumferential surface of the driving rod 103 (shown by an arrow A), and therefore a condition of contact between the pinch member 106 and the driving rod 103 is made unstable in a state in which the movable body 108 is mounted on the driving rod 103. As a result, a variation is caused in the frictional force between the pinch member 106 and the driving rod 103, and between the slider 105 and the driving rod 103, among a plurality of driving devices having the same configuration. For that reason, it is essential and thus troublesome to inspect whether the frictional force for the movable body 108 is within a desired range with regard to each driving device 100 after the each driving device 100 is assembled.
In the driving device 100, the pinch member 106 is pressed against the driving rod 103 by the leaf spring 107. The leaf spring 107, however, is generally a biasing member having a large spring constant and causes the frictional force to enormously vary according to minute error of mounting, thus making a cause of large variation in driving force and driving velocity among a plurality of driving devices 100.
When the leaf spring 107 bent into the shape generally like the square bracket is mounted, the spring is expanded opposite to a direction in which the spring biases. Accordingly, the leaf spring 107 may sometimes be expanded beyond a yield point of the leaf spring 107 so that the spring constant may be changed.
The leaf spring 107 is produced with use of a press die. Therefore, a change of design of the leaf spring 107 requires adjustment of the die and thus results in cost increase. Besides, the leaf spring 107 that causes the frictional force equal to a designed value is difficult to produce with only one trial and often requires several trials.
In JP H08-70586 A, as shown in FIG. 5, a driving device is disclosed in which a pinch member 106 is pressed from right above by a coiled spring 109. In this device also, however, a position of the pinch member 106 is not restricted with respect to directions shown by an arrow A, and the problem of variation in frictional force is caused as described above.